The use of positive photoresists is generally well-known, and is described by Wayne M. Moreau in Semiconductor Lithoqraphy: Principles, Practices, and Materials, Plenum Press, New York, N.Y. (1988), especially in Chapter 2, "Positive Resists," at pages 29-80, which is incorporated herein by reference. The general structures and manufacturing processes for electronic packages, including the use of positive photoresists in the fabrication of electronic packages, are described in, for example, Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, N.Y., (1988), especially Chapter 12, "Lithography In Electronic Packaging," by Gerald W. Jones, Jane M. Shaw, and Donald E. Barr, at pages 372 to 379 thereof, and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), especially Chapter 12, "Printed Circuit Board Packaging" by Donald P. Seraphim, Donald E. Barr, William T. Chen, George P. Schmitt, and Rao R. Tummala, at pages 898 to 904 thereof, both of which are hereby incorporated herein by reference. PG,5
Positive photoresists are compositions that contain high molecular weight compounds that define a pattern for subsequent processing. In the case of integrated circuit chips, processing takes the form of metal evaporation, implantation, and silicon oxidation. In the case of printed circuit boards and ceramic packages the subsequent processing takes the forms of (1) selective addition of metals or other elements to the surface by plating, or (2) the subtraction of metals or other elements from the surface by one form or another of etching. The parent polymer of the resist protects the material underneath the resist pattern from these processing steps.
In photolithography, images are created in the photoresist by exposure to light through a mask, with subsequent developing to form a pattern of open (exposed) areas and closed (protected) areas. In this way the resist transforms the two dimensional circuit design on a mask into a three dimensional circuit feature. Where small features, for example on the order of 1 to 15 microns, are imaged, positive photoresists are especially preferred. Positive photomasks are mostly opaque and are less subject to particle defects. Additionally, the high contrast exhibited by positive resists makes them especially useful for laser direct write production of circuit boards. Positive photoresists are characterized by an increase in solubility upon exposure to light. This is contrary to negative resists which become less soluble upon exposure to actinic radiation.
Photoacids work by the photolytic formation of Lewis Acids and protonic acids. These photo generated acids catalyze the deprotection of the acid labile groups or degrade the main chain by acidolysis. The action of the acid may be one or both of main chain scission or side chain scission. The preferred photo acid generators have heretofore been onium salts, that is, salts of the type ArN.sub.2 X, Ar.sub.2 IX, and Ar.sub.3 SX, where Ar is an aryl group, and X is typically BF.sub.4 -, PF.sub.6 -, AsF.sub.6 -, and SbF.sub.6 -. The anions represented by X must be disposed of after development, and salts of the type Ar.sub.2 IX exhibit incompatibility with copper and copper containing substrates. This precludes the use of Ar.sub.2 IX salts in circuit board production. Thus, it is an object of the invention to overcome these shortcomings of the present technology, and avoid the use of Ar.sub.2 IX and the formation of such anions as BF.sub.4 -, PF.sub.6 -, AsF.sub.6 -, and SbF.sub.6 -.
Photoacid generators and photoresist systems are described in U.S. Pat. No. 4,618,564 to Christopher G. Demmer and Edward G. Irving for Process for Production of Positive Images Using Sulfonic Acid Precursors, U.S. Pat. No. 4,371,605 to Carl A. Renner for Photopolymerizable Compositions Containing N-Hydroxyamide and N-Hydroxyimide Sulfonates, U.S. Pat. No. 4,258,121 to Teruo Kojima for Photopolymerizable Compositions, and U.S. Pat. No. 4,425,424, for Dye-Forming Compositions. These patents all show photoacid generators of the type ##STR1## where R.sup.1 may be an aryl group and R.sup.2 may be an aryl or alkyl group.
U.S. Pat. No. 4,618,564 to Christopher G. Demmer and Edward G. Irving for Process for Production of Positive Images Using Sulfonic Acid Precursors describes a positive photoresist system where the dissolution of the positive resist composition is inhibited by the acid generator in an aqueous base. The acid generator, e.g., a sulfonic acid generator, forms an acid and thereby enhances the rate of solubilization of the polymer in areas exposed to actinic radiation. However, Demmer et al. do not use chemical amplification. As a consequence, Demmer et al. use amine containing photosensitizers, as crystal violet, which are not only incompatible with chemically amplified systems, but actually impair the photospeed thereof. Moreover, because the Demmer et al. system is not a chemically amplified system, actinic energy doses approaching 1 Joule/cm.sup.2 are required to effect solubilization of the positive resist.
The other three patents all pertain to negative working resists, in which a monomer polymerizes or crosslinks with a matrix to form an insoluble film. U.S. Pat. No. 4,371,605 to Carl A. Renner for Photopolymerizable Compositions Containing N-Hydroxyamide and N-Hydroxyimide Sulfonates, U.S. Pat. No. 4,258,121 to Teruo Kojima for Photopolymerizable Compositions, and U.S. Pat. No. 4,425,424, for Dye-Forming Compositions, describe the use of this type of acid generator in acid catalyzed photopolymerization. These three patents all describe the use of toxic and/or flammable solvents to process, that is, to develop the image.
U.S. Pat. No. 4,491,628 to Hiroshi Ito, Carlton G. Willson, and Jean M. J. Frechet for Positive- and Negative-Working Resist Compositions With Acid Generating Photoinitiator and Polymer With Acid Labile Groups Pendant From Polymer Backbone describes a deep ultraviolet resist which works in the 240 to 300 nanometer region. The described resist systems are formulated from polymers having recurrent pendant groups, such as t-butyl esters or t-butyl carbonates, that undergo efficient acidolysis with concommitant changes in solubility when an onium salt, for example, azodiazonium salts, diaryliodonium salts, or triarylsulfonium salts, are exposed to actinic radiation, as ultraviolet, electron beam, or x-ray radiation. While most onium salts can not be efficiently sensitized to wavelengths longer than 435 nanometers, diaryliodonium salts can be efficiently sensitized to the longer wavelengths. However, an interaction between iodonium salts and copper precludes the use of iodonium salts for printed circuit boards.